Sheet stacker



FIG. 1 is a left-front perspective view of a sheet stacker showing the new design;

FIG. 2 is a front view thereof;

FIG. 3 is a left side view thereof;

FIG. 4 is a right side view thereof;

FIG. 5 is a top view thereof; and,

FIG. 6 is a rear view thereof.

The broken lines showing in FIGS. 1-6 are for illustrative purposes only and form no part of the claimed design. 

The ornamental design for a sheet stacker, as shown and described. 